1. Field of the Invention
This invention relates to a parison and a container obtained by biaxial stretch-blow molding of the parison and to processes for production thereof. More specifically, it relates to a biaxially stretched blow molded container having excellent transparency, gas-barrier property and mechanical strength, and a parison which is a precursor of the container, and to processes for production thereof.
2. Description of the Prior Art
Glass containers have been widely used heretofore for holding a variety of goods, for example carbonated drinks, beer, wines and liquors, seasonings, oils, cosmetics, and detergents. The glass containers, however, are defective in regard to handling, safety, and the cost of production.
In the field of containers for seasonings, carbonated drinks, detergents and cosmetics which require transparency, biaxially stretched blow-molded containers of various resins, mainly polyethylene terephthalate (PET for short), have been widely used.
Biaxially oriented containers of PET do not necessarily have perfect properties for packaging purposes. When, for example, foods and beverages which require a high level of gas barrier property are held in such containers, the flavor of the contents will be spoiled because these resins do not possess sufficient oxygen and carbon dioxide gas barrier properties.
Various methods are known for the production of multilayered containers having gas-barrier properties.
For example, there was proposed a method of producing a multilayered container, which comprises using an injection-molding machine having an injection cylinder for a thermoplastic polyester resin and an injection cylinder for a meta-xylylene group-containing polyamide resin ("MX nylon" for short) as a thermoplastic gas barrier resin, and sequentially injecting the thermoplastic polyester resin and then the MX nylon from these separate injection cylinders into a single mold by one mold closing action to form a three-layered structure composed of an inside layer of the thermoplastic polyester resin, an outside layer of the thermoplastic polyester resin and a core layer of the MX nylon (Japanese Laid-Open Patent Publications Nos. 128516/1982 and 128520/1982; and the corresponding U.S. Pat. No. 4,535,901). If in this method, the amount of the MX nylon to be injected is decreased in an attempt to reduce the thickness of the core layer, the core layer of MX nylon cannot be formed entirely on the body portion.
There was also proposed a method in which a thermoplastic polyester resin, MX nylon and the thermoplastic polyester resin are injected sequentially in this order to form a five-layered structure with three layers of the thermoplastic polyester resin and two layers of MX nylon being laminated alternatively, and as a result, in spite of the two layers of MX nylon, the thickness of the MX nylon layers can be very much reduced and the amount of MX nylon injected can be decreased from that in the aforesaid method (Japanese Laid-Open Patent Publication Ser. No. 240409/1980 and corresponding U.S. Pat. application Ser. No. 87969 and European Patent Publication Ser. No. 161625; Japanese Laid-Open Patent Publication Ser. No. 108542/1986 and corresponding U.S. Pat. application Ser. No. 793095 and European Patent Publication Ser. No. 180,191).
Japanese Patent Publication Ser. No. 16326/1985 (corresponding to U.S. Pat. No. 4,174,413) discloses a method by which the amount of a gas-barrier resin injected can be decreased. In this method, part of a thermoplastic polyester resin is first injected, and then the remainder of the thermoplastic polyester resin and the gas-barrier resin are simultaneously injected. The product is of a three-layer structure in section in which the gas-barrier layer is shifted toward one side from the center.
Generally, the adhesion between PET and a resin having excellent gas-barrier property such as a m-xylylene group-containing polyamide resin or an ethylene/vinyl acetate copolymer is very poor, and the delamination resistance between PET and the gas-barrier resin layer is low. Consequently, delamination tends to occur between these resin layers when an external deforming force or an impact force is exerted on the container or an internal pressure is exerted as in a container filled with a solution containing carbon dioxide gas. The delaminated portion of the container separates into two films and looks slightly white. Hence, the appearance of the container is undesirable.
On the other hand, gas-barrier containers having a layer of a mixture of a gas-barrier resin and a thermoplastic polyester layer have also been proposed. For example, there have been proposed containers composed only of such a mixed resin layer (Japanese Laid-Open Patent Publications Ser. Nos. 90033/1983 and 160344/1983), and a multilayered container containing the above mixed barrier layer for preventing delamination between resin layers (Japanese Laid-Open Patent Publication Ser. No. 183243/1983 corresponding to U.S. Pat. No. 4,501,781). In the former containers, if the mixed layer (to be referred to as the uniformly mixed resin layer) results from sufficient mixing (to be referred to as uniform mixing) of two or more resins, haze occurs to reduce the transparency of the containers and therefore their merchandise value. In the latter container, delamination resistance between the gas-barrier layer and the PET layer increases, but haze occurs in the part of the uniformly mixed resin layer obtained by mixing in a usual manner, leading to a loss of transparency.
In the case of uniform mixing by a usual method, if the proportion of the gas-barrier resin in the uniformly mixed resin layer is increased, the resulting container becomes hazy in opalescent or pearly color, and no adhesion strength increases. On the other hand, if the proportion of the thermoplastic polyester resin in the uniformly mixed resin layer is increased, delamination resistance increases, but haze occurs in opalescent or pearly color.
We have extensively worked on this problem, and now found that in the production of a container by using a mixture of a thermoplastic resin (resin A) and another thermoplastic resin having gas-barrier property (resin B), the problem of haze depends upon the state of mixing the resins, and that this problem can be solved by mixing the resins in a specific multilayered state or a specific mixed state.